1. Field of the Invention
The present invention relates to a fuel cell having circular disk-shaped electrolyte electrode assemblies interposed between separators. Each of the electrolyte electrode assemblies includes an anode, and a cathode, and an electrolyte interposed between the anode and the cathode. Further, the present invention also relates to a fuel cell stack formed by stacking a plurality of such fuel cells.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates), and the electrolyte electrode assembly and the separators make up a unit of fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the anode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce H2O or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric current.
Generally, the solid oxide fuel cell is operated at a high temperature in the range from 800° C. to 1000° C. The solid oxide fuel cell utilizes the high temperature waste heat for internal reforming to produce the fuel gas, and generates electricity by spinning a gas turbine. The solid oxide fuel cell is attractive as it has the highest efficiency in generating electricity in comparison with other types of fuel cells, and receiving growing attention for potential use in vehicles in addition to the applications in combination with the gas turbine.
Stabilized zironia has a low ion conductivity. Therefore, the electrolyte membrane formed of stabilized zirconia needs to be thin so that oxygen ions move through the electrolyte membrane smoothly for improving the power generation performance. However, the electrolyte membrane of the stabilized zirconia can not be very thin for maintaining the sufficient mechanical strength. Therefore, it is difficult to produce a large electricity using the membrane of stabilized zirconia in the solid oxide fuel cell.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 5-266910 (prior art 1) discloses a solid oxide fuel cell system in which a plurality of cells are disposed on one surface (area) between adjacent separators. In the prior art 1, the plurality of cells are provided between the separators to increase the total surface area of the cells for generating a large current, while preventing damages to the electrolyte plate to improve the reliability of the fuel cell system.
FIG. 25 is a perspective view showing the fuel cell system disclosed in the prior art 1. As shown in FIG. 25, the fuel cell system includes a plurality of layers stacked together to form a stack body. Each of the layers includes a separator 1 and four cells 2 placed on the separator 1. A fuel gas plate 3 on the lowermost layer has supply ports and discharge ports for supplying and discharging a fuel gas. An oxygen-containing gas plate 4 on the uppermost layer has supply ports and discharge ports for supplying and discharging an oxygen-containing gas.
Fuel gas supply manifolds 5a, 5b extend through the separators 1 for supplying the fuel gas to each of the cells 2, and fuel gas discharge manifolds 5c, 5d extend through the separators 1 for discharging the fuel gas from each of the cells 2 after reaction. Further, oxygen-containing gas supply manifolds 6a, 6b extend through the separators 1 for supplying the oxygen-containing gas to each of the cells 2, and oxygen-containing gas discharge manifolds 6c, 6d extend through the separators 1 for discharging the oxygen-containing gas from each of the cells 2 after reaction.
The fuel gas supply manifolds 5a, 5b are connected to fuel gas supply pipes 7a, 7b at the fuel gas plate 3. The fuel gas discharge manifolds 5c, 5d are connected to fuel gas discharge pipes 7c, 7d at the fuel gas plate 3. The oxygen-containing gas supply manifolds 6a, 6b are connected to oxygen-containing gas supply pipes 8a, 8b at the oxygen-containing gas plate 4. The oxygen-containing gas discharge manifolds 6c, 6d are connected to oxygen-containing gas discharge pipes 8c, 8d at the oxygen-containing gas plate 4.
For example, in the fuel gas plate 3 of the fuel cell system, the fuel gas supplied to the fuel gas supply pipes 7a, 7b flows into the fuel gas supply manifolds 5a, 5b of the separators 1 in the stacking direction and the fuel gas is supplied to the anode of each cell 2. After the reaction at the anode, the fuel gas flows back to the fuel gas plate 3 through the fuel gas discharge manifolds 5c, 5d, flows into the fuel gas discharge pipes 7c, 7d, and is discharged to the outside of the fuel cell system. In the oxygen-containing gas plate 4, in the similar manner, the oxygen-containing gas is supplied to, and discharged from the fuel cell system through the oxygen-containing gas plate 4.
As described above, the fuel gas supplied to the fuel gas plate 3 and the oxygen-containing gas supplied to the oxygen-containing gas plate 4 flow through the separators 1, and supplied to four cells 2 on each of the separators 1. Therefore, the sealing structures for preventing the leakage of the reactant gases (fuel gas and oxygen-containing gas) are required for the separators 1 (one sealing structure is needed for every four cells 2). The sealing structures are considerably complicated in the fuel cell system.
The fuel gas plate 3 is connected to the fuel gas supply pipes 7a, 7b, and the fuel gas discharge pipes 7c, 7d. The oxygen-containing gas plate 4 is connected to the oxygen-containing gas supply pipes 8a, 8b, and the oxygen-containing gas discharge pipes 8c, 8d. Therefore, the overall fuel cell system is considerably large.
Further, Japanese Laid-Open Patent Publication No. 6-310164 (prior art 2) discloses another type of solid oxide fuel cell. In the solid oxide fuel cell, a plurality of unit cells each having a small surface area are provided on each of metallic separators, and a fuel gas supply hole and an oxygen-containing gas supply hole are formed centrally in each of the unit cells. The prior art 2 is directed to provide a fuel cell system having an improved reliability in which the total surface area of the cells on the separator is large, and the substrate is crack-free.
However, in the prior art 2, the unit cells may not be positioned at predetermined positions accurately. The fuel gas supply hole and the oxygen-containing gas supply hole provided centrally in each of the unit cells need to be accurately in alignment with a fuel gas supply manifold and an oxygen-containing gas supply manifold of the separator. The positioning operation is very difficult. Thus, the assembling operation of the fuel cell is laborious, and the production efficiency of the fuel cell is low.
Japanese Laid-Open Patent Publication No. 7-122287 (prior art 3) discloses an inside manifold system sheet type solid oxide fuel cell module. Gas separating plates are disposed at the upper end and lower end of a fuel cell stack. A plate made of the same material as that of the gas separating plates is provided outside of at least one of the gas separating plates. An insulative side surface supporting member for supporting side surface supporting member for supporting side surfaces of the fuel cell stack extends for each side surface of the cell stack. One end of the insulative side surface supporting member is joined to the plate.
However, the prior art 3 is directed to the prevention of misalignment of the cells and separating plates in the horizontal direction. Therefore, the prior art 3 does not enable plurality of cells to be positioned accurately on the separator surface.